1. Field of the Invention
The present invention relates to an ink container, an inkjet printing apparatus, a printing head, and an ink supplying method and, more particularly, the invention is preferably applied to an inkjet printing apparatus in which ink is intermittently supplied to a printing head for ejecting ink.
2. Description of the Related Art
Inkjet printing apparatuses which form an image on a printing medium by depositing ink to the printing medium using an inkjet printing head include that which forms an image by ejecting ink while moving a printing head relative to a printing medium and that which form an image by ejecting ink while moving a printing medium relative to a fixed printing head conversely.
There are two general types of methods of supplying ink to a printing head used in such an inkjet printing apparatus. One is a type in which a supply system is configured such that an amount of ink is always or continuously supplied to a printing head according to the amount of ink ejected (hereinafter referred to as a continuous supply type), and the other is a type in which a printing head is provided with a reservoir (sub-tank or second ink tank) for reserving a predetermined amount of ink and in which a supply system is configured such that ink is supplied to the reservoir from an ink supply source (main tank or first ink tank) at appropriate timing or intermittently (hereinafter referred to as an intermittent supply type).
The continuous supply type is further categorized into two types, for example, when it is used in an inkjet printing apparatus of a type referred to as a serial type in which a printing head is scanned back and forth in predetermined directions relative to a printing medium and in which the printing medium is transported in a direction substantially orthogonal thereto to form an image. One is a type referred to as an on-carriage type in which ink is supplied by integrally or detachably attaching an ink tank to a printing head that is carried and moved back and forth (main scanning) by a carriage. The other is a tube supply type in which an ink tank that is separate from a printing head carried on a carriage is fixedly installed in a part of a printing apparatus other than the printing head and in which the ink tank is connected to the printing head through a flexible tube to supply ink. In some of the latter type, a second ink tank that serves as an intermediate tank between an ink tank and a printing head is mounted on the printing head or the carriage.
When an on-carriage type structure is adopted, there are limits on the project area in a direction perpendicular to the main scanning direction and volume of members that move with a carriage (a printing head and an ink tank undetachably or detachably integrated with the same). Therefore, only an ink tank having a very limited capacity can be used when a small-sized printing apparatus, especially, a portable printing apparatus is to be formed. This results in very frequent replacement of the printing head integral with the ink tank or the ink tank alone, which has been problematic from the viewpoint of operability and running cost. Further, the recent spread of so-called mobile apparatus is remarkable and, for example, ultra-compact inkjet printers have been proposed which can be integrated with notebook type personal computers and digital cameras. It is considered impractical to design such printers in adaptation to the on-carriage method.
When a tube supply type structure is adopted, although members that move with a carriage during main scanning can be made compact to some degree, it is difficult to make the apparatus as a whole compact because a space is required for a tube member to move to follow up the carriage, the tube member coupling a printing head on the carriage and an ink tank located outside the carriage to supply ink. Further, the recent trend is that a carriage is scanned at a high speed to accommodate increases in the speed of printing operations, and resultant severe rocking of a tube that follows the carriage results in changes in the pressure of ink in an ink supply system for the printing head. It is therefore required to provide various complicated pressure buffering mechanisms in order to suppress pressure changes, it has been difficult to achieve a size reduction in this respect too.
On the contrary, in the case of the intermittent supply method that is used for serial type inkjet printing apparatus for example, a relatively small second ink tank and printing head are provided on a carriage; a relatively large first ink tank is provided in a part other than the carriage of the printing apparatus; and a supply system is configured such that ink is supplied from the first ink tank to the second ink tank at appropriate timing. A structure is also employed in which the ink supply system between the first and second ink tanks is spatially separated or the ink channel is blocked with a valve during main scanning to achieve fluid isolation between the first and second ink tanks. Basically, this makes it possible to solve various problems attributable to the size of moving members as described above such as an ink tank and the rocking of a tube that have limited efforts to achieve a small size in the case of the continuous supply type.
When an intermittent supply type structure is adopted, however, it is important to discharge a gas such as air that enters or has entered an ink supply system and to control the pressure inside the same properly.
There are four general causes for the entrance of a gas into a supply system. 1) A gas can enter through ink ejection openings of a printing head or can generate as a result of an ejecting operation. 2) A gas that has been dissolved in ink can be separated from the same. 3) A gas can enter a supply system from the outside through the material from which the supply system is formed as a result of permeation. 4) A gas can enter when a joint is coupled to couple a first ink tank and a second ink tank.
The entrance of a gas is a problem that inevitably occurs, although the amount of the gas varies depending on the structure of the supply system. When a gas is accumulated in a second ink tank on a carriage for example, a problem arises in that the efficiency of charging the second ink tank with ink is reduced accordingly. Further, unexpected pressure changes are caused by expansion and contraction of air in response to temperature changes. This can result in leakage of ink from ejection openings attributable to an action of a resultant excessively large positive pressure or can conversely result in a failure of ink ejection attributable to an action of an excessively large negative pressure. Furthermore, the gas accumulated in the second ink tank can be included in ink that is guided to the ejection openings to cause problems such as disablement of ink ejection.
Such problems can be similarly caused when a continuous supply system of the tube supply type is configured. In a tube supply type continuous supply system in the related art, measures have been taken against such entrance of a gas, including a recovery operation for discharging ink and the gas from the printing head by simultaneously sucking them through the ejection openings thereof periodically or forcibly and a recovery operation performed in case that a second ink tank is carried by the carriage in which the gas is forcibly discharged from the second ink tank along with ink concurrently with an operation of sucking them through the ejection openings.
Since a great amount of waste ink is generated as a result of the adoption of such measures, serious limits are put on designing when a compact and portable printing apparatus is to be provided using the intermittent supply method. Further, a long time must be included in a control sequence of the printing apparatus to accommodate at least a recovery operation for sucking ink from the ejection openings of the printing head in addition to an operation of filling the second ink tank with ink at appropriate timing. In addition, since it is also required to perform a wiping operation for removing ink deposited on the surface of the printing head having the ejection openings formed thereon as a post-process for the recovery operation and a preliminary ejecting operation, a problem arises in that a further time is spent accordingly.
Referring to the continuous supply system of the tube supply type, in the case of an inkjet printing apparatus in which a pressure that is negative relative to the atmosphere must be generated to hold ink meniscuses formed at the ejection openings, there are limits including a need for providing the first ink tank in a position lower than the position of the ejection openings of the printing head in order to generate a negative pressure in the first ink tank naturally. This puts a limit on even the position and attitude or orientation of the ink tank and has resulted in problems including leakage of ink from the ejection openings especially in case that a portable printing apparatus is to be provided which is unstable in attitude during transportation.
On the contrary, proposals have been made for the adoption of the intermittent supply system, including a proposal in which a film having a function of allowing a gas to pass while disallowing a liquid to pass (hereinafter simply referred to as a functional film) is disposed to separately discharge only a gas from the second ink tank by force through the functional film and in which a porous member such as a sponge for holding ink is contained in the second ink tank to generate an adequate negative pressure therein. Such a structure is advantageous for even a portable printing apparatus whose attitude is unstable during transportation because it effectively suppresses an increase in the amount of waste ink generated during when ink is charged.
However, in order to use the functional film with stability, it is required that the film stays in a chemically inert state for a long time, which has resulted in a problem in that freedom in selecting ink is reduced, e.g., ink having a composition that does not affect the functional film must be selected.
When the functional film is provided on the second ink tank, a gas can conversely flow in the direction of entering the second ink tank. When a negative pressure generating mechanism such as a porous member for keeping ink under a negative pressure relative to a nozzle of the printing head is provided in the second ink tank for this reason, the efficiency of containing ink in the second ink tank is limited. Designing may be limited with respect to deposition of dyes and pigments in ink and endurance of the porous member against deterioration, which also reduces alternatives and freedom in selecting ink.
Further, in such a structure, since the porous member is always over-charged with ink when ink charging is completed, the over-charged ink in the porous member must be discharged as waste ink without fail by performing an operation of sucking the printing head through the ejection openings after the charging is completed in order to apply a required negative pressure to the printing head. That is, a problem arises in that a charging operation is accompanied by the generation of waste ink.